1. Field of the Invention
The present invention relates to a bonding wire for electrically connecting a semiconductor chip and a lead frame or a printed circuit board (PCB) in a packaging process, and more particularly, to a bonding wire which does not cause chip cracks because it forms a soft ball.
2. Description of the Related Art
Referring to FIG. 1, a general semiconductor package includes a semiconductor chip 10 on a pad 50, a plurality of chip pads 20 formed on the semiconductor chip 10 as input/output ports for a variety of signals, a plurality of lead frames 40 electrically connected to the semiconductor chip 10 to receive the variety of signals from or to output the same to an external circuit; and a bonding wire 30 for electrically connecting the chip pad 20 and the lead frame 40. This structure of the general semiconductor package is protected by, for example, epoxy molding compound resin.
One end of the bonding wire 30 bound to the chip pad 20 includes a compressed ball 32 and a neck 34 as a connector between the compressed ball 32 and the bonding wire 30. In a wire bonding process, one end of the bonding wire 30 is melted by discharging to form a free air ball of a predetermined size and pressed on the chip pad 20 to be bound to the chip pad 20. A loop of the bonding wire 30 having an appropriate height and length is formed to reach a corresponding lead frame 40, and the other end of the bonding wire 30 is bound to the lead frame 40 with an application of pressure. As a result, the semiconductor chip 10 and the lead frame 40 are electrically connected.
A gold-silver (Au-Ag) alloy is typically used for the bonding wire 30. Silver forms a complete solid solution together with gold, so the alloy can be produced on a large scale, thereby lowering the manufacturing cost of the bonding wire. However, when a loop is formed with the conventional bonding wire 30, the bonding wire 30 often breaks near the neck 53, as indicated by reference numeral 35. A reduced loop height due to the increasing integration density of a semiconductor device further increases the breakage 35 of the neck 34. The breakage 35 occurs due to a poor tensile strength of the bonding wire 30. Accordingly, a signal transmission path formed by the loop is opened. In addition, the poor strength of the bonding wire 30 causes a loop sagging or sweeping (indicated by reference numeral 36) in a molding process, so that adjacent loops contact one another.
Examples of research on the conventional Au-Ag bonding wire are in Japanese Laid-open Application Nos. 1998-326803, 1999-6781 1, and 1999-67812. According to the disclosures, to increase the bonding strength and to reduce the sweeping of the Au-Ag bonding wire in the molding process, cupper (Cu), palladium (Pd), or platinum (Pt) was added in an amount of 0.01-4% by weight, and at least one of calcium (Ca), beryllium (Be), and indium (In) was added in an amount of 0.0005-0.05% by weight. Japanese Laid-open Patent Application No. 2000-150562 disclosed that the addition of at least one of Pd, titanium (Ti), Ca, Be and lanthanum (La) in an amount of several parts per million to several percentages by weight results in a ball grain refinement and increases bonding strength after high-temperature heating.
However, the conventional techniques were inclined to increasing the bonding strength and reliability and to preventing the sweeping of the bonding wire in the molding process, ignoring a crucial chip crack due to the inherently hard ball of the Au-Ag alloy. In addition, the problem of the breakage occurring near the ball neck in the loop formation remains unsolved. Furthermore, the ball grain refinement in Japanese Laid-open Patent Application No. 2000-150562 increases the area of grain boundaries acting as Au and Ag atom diffusion paths, and thus it is likely to lower reliability.
Accordingly, it is an object of the present invention to provide a highly reliable bonding wire which has a strong tensile strength at both room temperature and high-temperature and causes no chip crack in a wire bonding process and no rupture of a ball neck when the bonding wire is looped.
In one aspect, the present invention provides a bonding wire for a semiconductor device, containing: a Au-Ag alloy including about 5-40% Ag by weight in Au having a purity of 99.999% or greater; at least one element of a first group consisting of Pd, Rh, Pt, and Ir in an amount of about 50-10,000 ppm by weight; at least one element of a second group consisting of B, Be, and Ca in an amount of about 1-50 ppm by weight; at least one element of a third group consisting of P, Sb, and Bi in an amount of about 1-50 ppm by weight; and at least one element of a fourth group consisting of Mg, TI, Zn, and Sn in an amount of about 5-50 ppm by weight. In the bonding wire according to the present invention, as many kinds of elements as possible from each of the first through fourth groups are added for synergism.
In the specification, the amounts of elements are expressed in ppm by weight and % by weight based on the total weight of the bonding wire.